In a code division, multiple access (CDMA) communications system, a plurality of user communication signals can be transmitted within and thus share the same portion of the frequency spectrum. This is accomplished by providing a plurality of different pseudonoise (PN) binary code sequences (e.g., one for each user) that modulate a carrier, thereby "spreading" the spectrum of the resulting waveform. In a given receiver, all of the user signals are received, and one is selected by applying an assigned one of the PN binary code sequences to a correlator to extract only the signal energy intended for the receiver, thereby "despreading" the received CDMA transmission. All other (uncorrelated) user transmissions appear as noise.
In digital spread spectrum communications, data signals such as voice signals are digitized (turned into ones and zeros, or the like) and then there is impressed upon the digitized data a pseudonoise (PN) code, also referred to as a signature sequence. A pseudonoise code is usually a high frequency noise-like waveform that is multiplied with the digitized data before it is transmitted; this has the effect of spreading out the spectrum of the signal, hence the term spread spectrum communications. The spread digitized signal is then transmitted to a receiver, at which the same or a corresponding pseudonoise binary code sequence is applied to the received signal to despread it and extract the digitized signal. The digitized signal can then be digital-to-analog converted to obtain the original voice or other data signal. In a multi-user system, if each user in the system uses a different or distinct pseudonoise or signature sequence code, then at the other end of the link, if that particular noise-like waveform or signature sequence is applied to the received signal, then the data can then be extracted from that one user's signal, and any other user that is using a different noise-like or pseudonoise sequence will look just like background noise.
There are a number of approaches to accomplish such multi-user channelization. In one such approach, frequency division multiple access (FDMA), each user's transmitter has a distinct band of frequency, and the users do not overlap in frequency. The users can thus be distinguished by tuning to the appropriate frequency band. In time division multiple access (TDMA), every user's transmitter gets a specified time slot; all users then share and utilize the entire bandwidth of the selected channel, but each user transmits for only a short period of time and then turns off to let another user turn on. Thus, in TDMA, there is a series of users that take turns one at a time in a round robin fashion sharing the channel. In code division multiple access (CDMA), all of the users transmit all of the time, and can use the entire frequency band, so that the users can overlap both in frequency and in time in the resulting aggregate signal. In CDMA, the different users are identified or distinguished using pseudonoise codes or signature sequences. Every user is given a distinct pseudonoise code or signature sequence. The aggregate signal is the sum of all of the transmitted signals from all of the users with all of the distinct codes and distinct data. As long as the receiver knows the PN code of the user whose signal he is trying to extract from the aggregate signal that he receives, that receiver can then pull out the signal that it is interested in by knowing the PN code and using that PN code to accomplish despreading. At the modulator for each CDMA user, the signal to be transmitted is spread using the PN code, and then at the demodulator that signal is despread by multiplying the aggregate signal by the same PN code that was used at the transmitter.
In an asynchronous CDMA system, a user (also referred to as a subscriber unit) would transmit whenever it wants to, and the receiver (also referred to as a base station) would align its receiver to that incoming signal. Because the subscriber units in an asynchronous CDMA system do not try to coordinate their transmissions, but transmit whenever they want to, the base station will have to align itself with a distinct alignment for each active user's signal. Thus, when the base station is trying to despread each user's signal individually, it will have a different timing offset for every user. On the other hand, in a synchronous CDMA system, signals in the reverse channel (from subscriber unit to base station) are required to arrive with a particular phase alignment. Synchronous CDMA systems are further described in U.S. Pat. No. 5,499,236 issued Mar. 12, 1996 for "Synchronous Multipoint-to-Point CDMA Communication System" by Thomas R. Giallorenzi et al., and in its division U.S. Pat. No. 5,583,853 issued Dec. 10, 1996 for "Synchronous CDMA Transmitter/Receiver" by Thomas R. Giallorenzi et al., each of which is hereby incorporated by reference herein.
In a synchronous CDMA system, it is necessary for the base unit to be synchronized to the incoming signal, yet the delay or the time offset of the incoming signal from any particular subscriber unit is unknown initially because the signals are generated by subscriber units located at unknown respective distances from the base unit, the exact propagation time from each subscriber unit to the base unit is unknown, and the phase alignment of the clock in the subscriber unit that generated a particular signal is also unknown. Accordingly, it is necessary for the base station to initially synchronize itself to the subscriber unit signal or vice-versa. One approach to doing so is by having the base station staggering its clock so that it will align itself in time with the incoming signal. However, the inaccuracy caused by this approach in accomplishing initial synchronization and acquisition is undesirable. Another approach is that of U.S. Pat. No. 5,446,727 issued Aug. 29, 1995 for "Method and Apparatus for Time Aligning Signals for Reception in a Code-Division Multiple Access Communication System" by Eugene J. Bruckert et al. In U.S. Pat. No. 5,446,727, a coherent reverse channel, a per-chip spreading function, orthogonal spreading functions and a time alignment of all traffic channels are implemented such that the main signal of each channel arrives at a base-station within a fraction of a chip of one another. With this, the orthogonality among all channels is supposedly maintained, and, when demodulated, supposedly all channels except the channel of interest provides a cross-correlation of substantially zero with respect to the remaining signals. In U.S. Pat. No. 5,446,727, a multiplicity of local despreaders, each delayed relative to each other, are apparently provided. The outputs of all of these receivers are examined, and the receiver with the most power at the output is assumed to be the one that is most accurately synchronized with the received signal. The offset of the received signal is estimated as being equal to the delay carried by that particular receiver component. However, accuracy of this system is determined by the number of such relatively delayed receivers that are used, and the number of delayed receivers used thereby increases the amount of hardware needed for synchronization. Thus, the approach of U.S. Pat. No. 5,446,727 can only estimate the phase offset to within an accuracy equal to the spacing of their receivers in time. Thus, the only way that an arbitrarily good offset estimate could be obtained using the approach of U.S. Pat. No. 5,446,727 is to have an arbitrarily large number of receivers dedicated to each user, which is impractical.
Thus, there is a need to estimate offset in phase, time and/or frequency between a signal produced by a transmitter in one location and the clock of a receiver in another location. The present invention fulfills this need.